This invention relates generally to apparatus for use in drilling oil wells, and more particularly, but not by way of limitation, to a sub for connection to a drill bit, said sub being provided with means for creating an upwardly swirling flow in the well annulus above the drill bit.
During normal drilling operations a drilling fluid generally referred to as drilling mud is pumped down an internal bore of the drill string and out through a plurality of orifices in the rotary drill bit to wash away cuttings and other debris at the interface of the drill bit with the underground formation. This drilling mud then flows upward through the annulus between the drill string and the well bore to carry the cuttings away from the drill bit. Often, if the hydraulic horsepower at the bit (i.e. the fluid flow rate and pressure) is not adequate the bit may ball up due to ground up cuttings sticking on and around the bit teeth. Balling up of the bit causes lower penetration rate, excessive drag, and possible blowout and damaged wellbore.
It is, therefore, important that the design of any drill bit and its associated apparatus provide for hydraulic flow across the interface of the drill bit and the underground formation of a nature sufficient to clean away the cuttings from that interface.
The drilling mud must, however, accomplish another very important task in addition to cleaning cuttings away from the drilling interface.
This second task may generally be described as blowout prevention. The underground formations penetrated by the well borehole often contain very high pressure fluids. If the pressure within the borehole where it intersects the formation is less than the pressure of the fluid in the formation then an uncontrolled blowout may occur wherein high pressure formation fluid flows rapidly into and up the borehole potentially causing damage to drilling equipment and injury to drilling personnel at the surface.
Such blowouts are prevented by maintaining a column of drilling mud within the borehole of sufficient density that the hydrostatic pressure in the borehole at the intersection with any given underground formation is greater than the formation fluid pressure. This difference between hydrostatic pressure in the borehole and formation fluid pressure is commonly called the pressure differential. This pressure differential is typically on the order of several hundred p.s.i., i.e. the hydrostatic pressure of drilling mud within the borehole is several hundred p.s.i. greater than the formation fluid pressure. The pressure differential may be as great as several thousand p.s.i.
The term "pressure differential" as generally utilized in the drilling industry refers to this difference between hydrostatic pressure in the borehole and formation fluid pressure just described. There is, however, another "pressure differential" which is of significance to the following disclosure, namely, the difference between the rock stress (compression stress within the rock formation) and the hydrostatic pressure in the borehole. For purposes of differentiation between the two concepts, the term "fluid pressure differential" is used throughout the remainder of this disclosure to refer to the difference between hydrostatic pressure in the borehole and formation fluid pressure, and the term "rock stress pressure differential" is used to refer to the difference between the rock stress and the hydrostatic pressure in the borehole.
The prior art has recognized that this fluid pressure differential has a detrimental side effect in that it causes a hold-down force on cuttings at the interface between the drill bit and the formation. This hinders the removal of the cuttings and contributes to the problem previously discussed of balling up of the drill bit. The cuttings which are held down by the fluid pressure differential are ground to a paste on top of the unbroken rock formation.
This balling up of the bit and presence of unremoved cuttings at the drilling interface greatly reduces the penetration rate (i.e. the speed at which the well borehole is drilled) as compared to the rate which could be achieved with more complete removal of cuttings.
A thorough summary of the work previously done in this field, which explains the above mentioned problems in greater detail, is found in "Bits Designed To Reduce Bottom-Hole Pressure While Drilling", a thesis submitted to the graduate faculty of the Louisiana State University, Department of Petroleum Engineering, by Mohamed Sadik Bizanti, in December, 1978.
That thesis analyzes several structures which have been proposed to eliminate this problem of pressure differential at the interface of the drill bit and the formation. Those structures are found in U.S. Pat. No. 2,946,565 to Williams; U.S. Pat. No. 4,022,285 to Frank; U.S. Pat. No. 3,958,651 to Young, and U.S. Pat. No. 3,923,109 to Williams, Jr.
U.S. Pat. No. 2,946,565 to Williams proposes a drilling sub having an upward opening annular sealing cup disposed thereabout for sealing against the borehole and supporting a column of fluid above the cup. A jet nozzle diverts a portion of the downward flowing drilling fluid from within the drill string out an upward directed orifice within a passage through the sub which communicates with the annulus both above and below the sealing cup to form a jet pump which reduces the pressure within the annulus below the sealing cup.
U.S. Pat. No. 4,022,285 to Frank proposes a bit incorporating one or more upward directed jet pumps which entirely support the column of drilling fluid surrounding the drill string and provide a dry borehole bottom at the bit-formation interface. The upward movement of the drilling fluid through the jet pump causes a suction which removes the cuttings from the interface. This requires that a relatively small clearance be maintained between the drill string and the borehole immediately above the bit so that the column of fluid thereabove can be supported.
U.S. Pat. No. 3,958,651 to Young proposes to use air rather than drilling mud to carry the cuttings away from the drill bit. The air flows downward through an intermediate annulus contained in the drill string and then back up a central passage. A portion of the downward flowing air is diverted upward into the central passage to provide a jetting effect to aid the upward flow of air.
U.S. Pat. No. 3,923,109 to Williams, Jr. proposes a plurality of horizontally oriented jets directed at the corner between the sidewall of the borehole and the bottom of the borehole to wash away cuttings and a plurality of upwardly directed jets for inducing upward flow of the cuttings.
Another structure similar to those just discussed, but not analyzed in the Bizanti Thesis, is U.S. Pat. No. 2,765,146 to E. B. Williams, Jr. That reference proposes a drilling sub which diverts a portion of the drilling fluid flowing down through the drill string to a radial passage and out an upwardly directed orifice into the annulus between the drill string and the borehole to increase the upward velocity of the drilling fluid in the annulus.